Process of treating austenitic chrome-nickel steel alloys



Patented Feb. 1, 1938 v PROCESS OF TREATING AUSTENITIO CHROLIE-NICKELSTEEL ALLOYS Julius r. Sachse, Philadelphia, Pa., assignor to TheMidvale Company, Philadelphia, la., a corporation of Delaware NoDrawing.

Application January SerlalNo. 119,265

3 Claims. (Cl. 148-12) The object ofmy invention. is the manufacture ofarticles of chrome-nickel austenitic steel which will be resistant tointercrystalline attack, commonly indicated by the Strauss test,afterthey within about a range of 1000 to 1500 or 1600 F. The processfinds its most-useful application in the treatment of the so-called18-8" chromenickel austenitic steel, within which may be included alloysteel having percentages of chromium and nickel somewhat greater orsmaller than the percentages specified. The process is also applicableto the manufacture of chromium-nickel steel whose chromium and nickelcontents vary rather widely from the precentages specified. The prob lemof manufacturing 18-8 chrome-nickel steel that will be resistant tocorrosive agents within the range of temperature specified is lessdifilcult of solution if the carbon content be relativelyhavebeen'heated in normal use to a temperature low, say. of the order of0.06%, but the added by chrome-nickel austenitic steel when manu-,

factured in accordance with conventional methods has been explained inprior'patents and publications and need not be herein discussed. It willsufilce to explainthe procedure embodying my invention and thevariations therefrom which are advisable or necessary with variations inthe carbon content. The technical effects which it is L believed myimproved process achieves will also 40 be explained, although it will beunderstood that such explanation is based upon microscopic examinationand may not be as accurate and complete as more exhaustiveinvestigations may disclose. That the process does render the steelalloy resistant to corrosive agents without development of brittlenessor loss of durability when heated to within the specified temperaturerange has been, however, conclusively demonstrated by the practical useof the process.

In the first step of my process, the alloy is heated .to a temperatureabove that at which carbon is in substantially complete solution in themetal. That temperature increases as the carbon content of the alloyincreases. For carbon as low as .06% in a steel alloy of 18% chromium,8%

'main in solution. If said alloy contains only .06%

nickel, this temperature is about 1650 F.; for a carbon content of .25%this temperature is between 1900 and 1950 F. The temperature to whichthe alloy must be heated in the first step of my process is thereforevariable, but ordinarily 5 it will sufilce to heat the billet to anormal hot rolling temperature, usually 1900" to 2100 F.

Immediately after heating to the temperature specified, or before thetemperature has dropped to a temperature at which a substantial portionof the carbon goes out of solution, the metal is subjected to normal hotworking, such as forging, pressing or rolling. The progress of reductionby hot working is, however, so timed that the piece, when it arrives atthe point of final re- 15 duction to finished size, is at a temperatureat which the larger part of the carbon is out of solution. .Thus,dependent usually upon the percen- I tage of carbon in the alloy, themetal is heated to a temperature the minimum of which could vary fromabout 1650 to -1950 Rand the hot working, starting at about thattemperature, is finished at a temperature which may vary within widelimits, say from about 1800" down to about 1050" F., but which must besubstantially below the temperature of the original heating. Thus, ii an18-8" chrome-nickel steel alloy contains .25% carbon, the hot workingoperation may be completed at below 1650 F., at which temperature about.06% carbon will be in solution; but it is preferable to'continue thehot working operation to a decidedly lower temperature when still lesscarbon will be in solution. At a, temperature below 1500 IE. only about.025% carbon would re- 35 carbon, the hot working operation, in order toeffect an improvement, must be completed at below 1500 F., at whichtemperature, as just stated, about .025% carbonwill be in solution.Working to tlie lower temperature has the further advantage that theprecipitated carbides will be more thoroughly broken up by a workingcontinued after their precipitation. In all cases, however, it isnecessary that the temperature at the completion of working shall be ator below that at which the larger proportion of the carbon is out ofsolution. In no case should the work- 'ing continue after thetemperature approaches 1000 F.

The effect of rolling or other working while carbon is beingprecipitated from solution is tobreak' up the continuity of the envelopeof carbide which forms normally about the individual,- crystals of themetal. 4,

After finishing the rolling or other working, the

baris cooled. Preferably, but not necessarily, it is allowed to cool, inthe air, to atmospheric temperature.

In this as rolled or as worked" condition the metal does not possess itsfull corrosion resistance but is disintegrated by attack of acids atgrain boundaries after being subjected to a temperature within the range1000 to 1500 F.

In order to render the alloy immune to such attack, it is reheated to atemperature below that at which any considerable proportion of carbidesis dissolved, held at that temperature for a material length of time,say from two hours to v ing being immaterial.

' since a high percentage of chromium permits a greater proportion ofcarbides out 01' solution without depleting the adjacent metal to apoint where it becomes susceptible to corrosion. The permissivetemperature of reheating may also vary with different carbon contentsand may be higher withhigher carbon chrome-nickel alloy steel than withlower carbon chrome-nickel alloy steel, because it takes a highertemperature or longer time to secure the redistribution of chromium tothe depleted areas when a larger amount of carbide has lowered to agreater degree the chromium in the adjacent metal. Preferably thetemperature of reheating of an alloy containing 25% carbon should not beabove about 1400 F. while with lower percentages of carbon down to .06%the temperature of reheating should, preferably, not be above the rangeof 1200-1300 F. The temperature of reheating may also vary with thetemperature at which the hot working was completed. Thus, if the hotworking were completed at a temperature of 1400" F., the

reheating temperature may be as low as 1100" F. or even lower. If thetemperature at which the hot working was finished were comparativelyhigh, say just below 1650 F., the reheating temperature should not bebelow about 1300 F. If this reheating temperature be as high as 1600 F.,the material will still be immunized but comparatively soft, which isdesirable for some purposes. The lower temperature will give as good orbetter immunization with higher physical properties.

The following table gives illustrative reheating temperatures. which in48 hours will produce full resistance to acid attack in metals, ofdifi'erent carbon contents, the rolling of which has been completed atthe temperatures given in the first horizontal column of figures:

Percent carbon 1450 F. 1565 F. 1680 F 1795" F The reheatinghas threeeffects, namely: (1) the more complete spheroidizatlon oi the carbides;(2) the more complete liberation from solution of carbon; and (3)diflusion of chromium into portions around the carbides from whichchromium has been extracted in the formation of the carbides.

Chromium-nickel austenitic steel, when processed by the above'method ofrolling and reheating, is immune to intercrystaliine attack by theStrauss reagent following a heating in the range of 1000 to 1600 F.

What I claim and desire to protect by Letters Patent is:

1. The herein described method of treating nickel-chromium-carbon steelalloys to render them resistant to corrosion after subjection to atemperature within a range of 1000 to 1600 R, which comprises heatingthe alloy to such temperature above 1650 F. that the carbon is insubstantially complete solid solution in the alloy,

then subjecting the alloy to a hot working operation through atemperature zone ranging from the temperature specified at the beginningof the working to such temperature below a maximum of 1650 F., at whichthe *larger percentage of carbon will be out of solution, then coolingthe alloy, then reheating the alloy to a temperature in excess of 1000F. but below 1800 F. and substantially below the temperature of thefirst heating, holding the alloy at that temperature for a substantialperiod of time, and then cooling to atmospheric temperature,

2. The herein described method of treating nickel-chromium-carbon steelalloys to render them resistant to corrosion after subjection to atemperature within a range of 1000 to 1600 F., which comprises heatingthe alloy to such temperature above 1650 F. that the carbon is insubstantially complete solid solution in the alloy,

then subjecting'the alloy to a hot working oper-.

ation through a temperature zone ranging from the temperature specifiedat the beginning of the working to a temperature between 1050" and 1500F. at which the larger percentageof carbon will be out 01 solution, thencooling the alloy, then reheating to a temperature in excess of 1000 F.but'below 1600 F., be ding the alloy at that temperature for asubstantial period of time, and then cooling to atmospheric temperature.

3. The. herein described method of treating nickel-chromium-carbon steelalloys to render them resistant to corrosion after subjection to atemperature within a range of 1000 to 1600 E, which comprises heatingthe alloy to such temperature above 1650 F. that the carbon is insubstantially complete solid solution in the alloy, then subjecting thealloy to a hot working operation through a temperature zone ranging fromthe temperature specified at the beginning of the working to atemperature between 1050 F. and 1500 F. at which the larger percentageof carbon will be out of solution, then cooling the alloy, thenreheating to a temperature in excess of l000 F. but below 1500 F.,holding the alloy at that temperature for a substantial period of time,and then cooling to atmospheric temperature.

JULIUS F. SACI-ISE.

